Lithium secondary battery

ABSTRACT

A lithium secondary battery which comprises a negative electrode, a positive electrode, and disposed therebetween a polymer electrolyte comprising an ionically conductive polymer. The polymer electrolyte has a two-layer structure composed of a positive-electrode-side layer and a negative-electrode-side layer. The polymer electrolyte on the positive-electrode side contains a nonaqueous electrolytic solution containing a lithium salt in a higher concentration than that in the polymer electrolyte on the negative-electrode side. The battery is improved in charge/discharge cycling characteristics and high-load discharge characteristics.

FIELD OF THE INVENTION

[0001] This invention relates to a lithium secondary battery using anion-conductive polymer. More specifically, it relates to a lithiumsecondary battery comprising an anode for lithium batteries, a cathodecontaining or capable of inclusion/release of lithium, and anion-conductive polymer layer sandwiched between the cathode and theanode.

BACKGROUND ART

[0002] Lithium secondary batteries relying on the electrochemicalreactions of

Li⁺+e⁻→Li

[0003] in the charge stage, and

Li→Li⁺+e⁻

[0004] in the discharge stage have been vigorously studied and developedas a power source of portable electronic instruments or electric motorcars because they have a very high energy density in theory compared toother batteries and thus allow to manufacture small size, light weightbatteries. The performance of portable electronic instruments is everincreasing in recent years and their power consumption is alsoincreasing concominately therewith. For use as a power source of theseinstruments in particular, the power source is required to have asatisfactory discharge characteristics even under heavy loads. Followinglithium batteries using an organic electrolyte solution referred to as“lithium ion battery”, studies on batteries using a lithiumion-conductive polymer functioning both as the organic electrolytesolution and also as a polymer separator of the prior art batteries arein progress. The lithium secondary battery using the lithiumion-conductive polymer is very attractive because of its remarkableadvantages such as possibility of making the battery smaller and thinnerin size and lighter in weight as well as leak free. Lithium secondarybatteries of this type now available in the market use a porous matrixof ion-conductive polymer impregnate with or retaining an organicelectrolyte solution (a solution of lithium salt in an aprotic polarsolvent) therein. However, leakage of the organic electrolyte solutionfrom the battery has not fully been prevented in various environments.

[0005] When metallic lithium is used as anode in the lithium secondarybattery, one problem is how the growth of lithium dendrite on the anodeis prevented for improving the charge-discharge cycle characteristics.Various studies have also been made in order to solve this problem.JP-A-6223877 and JP-A-8329983 are representative examples of suchstudies among them.

[0006] JP-A-6223877 proposes to provide a plurality of ion-conductivelayers having different lithium salt concentrations between the cathodeand the anode in order to prevent the growth of lithium dendrite on theanode. JP-A-8329983 proposes to provide a pair of electrolyte layersseparately on the cathode and the anode, respectively and to give higherion-conductivity to the layer on the anode than the other layer on thecathode in order to prevent internal short circuit from occurring due tothe growth of lithium dendrite. The object of these proposals is toprovide a lithium secondary battery having high reliability andexcellent cycle characteristics by preventing the internal shortcircuit.

[0007] JP-A-2000/106,212 proposes a lithium battery having improvedbattery performance upon high rate discharge. The battery includes threeseparate layers of an electrolyte gel in which at least one layer eitheron the cathode or anode is different in the composition of electrolytegel from the electrolyte gel in the separator such that theconcentration of lithium salt is always higher in the separator than inthe cathode and/or anode. This proposal, however requires to form threeelectrolyte layers independently on the two electrodes and the separatorformed by laminating the three layers together and necessarily resultsin increased number of interfaces between the electrolyte gel layersundesirably for decreasing the internal resistance within the batteryand also increased number of steps for manufacturing the battery.

[0008] Although the performance of lithium secondary battery isadvancing by the use of ion-conductive polymer in conjunction with theuse of improved ion-conductive polymer on the anode, furtherimprovements are still demanded in the various performance of lithiumsecondary battery such as charge-discharge cycle life, dischargecharacteristics at a high load and other properties. In addition, wehave conceived to decrease the number of interfaces between variouselectrolyte layers by essentially dispensing with an independent layerof ion-conductive polymer in the separator and decrease the internalresistance of the battery correspondingly.

DISCLOSURE OF THE INVENTION

[0009] The patent literature cited above addresses prevention ofinternal short circuit on the anode side. As a result of our studies onthe ion-conductive polymer on the anode side, we have reached thefollowing conclusion. It is very difficult to fully prevent the growthof lithium dendrite even when a lithium ion-conductive polymercontaining an organic electrolyte solution in the matrix thereof isused. Conversely, when the growth of lithium dendrite is fullyprevented, the anode activity and the discharge characteristics underheavy loads will decrease.

[0010] Since the growth of lithium dendrite cannot be prevented with100% probability solely by the improvement in the ion-conductive polymeron the anode side and in view of the fact that the ion-conductivepolymer on the cathode side has not been improved well to date, ourattention was drawn to the improvement of the ion-conductive polymer onthe cathode side.

[0011] Accordingly, the present invention provide a lithium secondarybattery comprising an anode for lithium batteries, a cathode containinglithium or capable of inclusion/release of lithium, and anion-conductive polymer layer disposed between the cathode and the anode.According to the present invention, the concentration of a lithium saltin a nonaqueous electrolyte solution retained in the matrix of saidion-conductive polymer is higher on the cathode side than on the anodeside.

[0012] The present invention provides a small size, light weight batteryhaving the following advantages over the prior art battery.

[0013] 1) It has high performance and a high energy density. Higherlithium salt concentration on the cathode side decreases the interfacialresistance and enables not only the charge-discharge cyclecharacteristics but the discharging characteristics under a high load tobe improved.

[0014] 2) A discrete layer of ion-conductive polymer in the separatormay be dispensed with by joining two sub-layers of the ion-conductivepolymer integrally formed on the anode and the cathode together directlyto decrease the number of interfaces and also decrease the internalresistance in the battery correspondingly.

[0015] 3) Decomposition of compounds contained in the ion-conductivelayer may be prevented by using a particulate graphite having amorphouscarbon attached on the surfaces thereof. This allows the battery to beleak-free and exhibit high long term reliability as well as high safety.

[0016] 4) The battery may be produced by a highly efficient process.This is accomplished either by casting a precursor monomer of theion-conductive polymer onto the cathode and the anode and crosslinkingthe electrolyte layer by heat or UV radiation, or by crosslinking theelectrolyte layer in the separator integrally with the electrolyte layeron either one of the cathode and the anode.

BRIEF DESCRIPTION OF DRAWINGS

[0017]FIG. 1 is a graph showing the discharge capacity of the batteryaccording to Example 1 of the present invention at different currentlevels in comparison with the battery of Comparative Example 1.

[0018]FIG. 2 is a graph showing the discharge curve of the battery ofExample 1 of the present invention at a constant current of 10 mA incomparison with the battery of Comparative Example 1.

[0019]FIG. 3 is a graph showing the results of repeated charge-dischargecycle test performed on the batteries of Examples 2 and 3 of the presentinvention in comparison with the battery of Comparative Example 2.

[0020]FIG. 4 is a graph showing the results of repeated charge-dischargecycle test performed on the battery of Example 4 of the presentinvention in comparison with the battery of Comparative Example 3.

[0021]FIG. 5 is a graph showing the discharge capacity of the battery ofExample 5 of the present invention at different current levels incomparison with the battery of Comparative Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] The present invention relates to a lithium secondary batterycomprising an anode for lithium battery, a cathode containing or capableof inclusion/release of lithium, and an ion-conductive polymer layerdisposed between the cathode and the anode.

[0023] According to the present invention, the ion-conductive polymerlayer is comprised of a matrix of said ion-conductive polymer retaininga nonaqueous electrolyte solution therein and comprises a laminate oftwo sub-layers formed on the cathode and the anode. The invention isfurther characterized in that the concentration of a lithium salt in thenonaqueous electrolyte solution is higher in the sub-layer on thecathode than in the sub-layer on the anode.

[0024] By setting the lithium salt concentration in the nonaqueouselectrolyte solution at different levels in this way, the followingadvantages are achieved in comparison with the case where the lithiumsalt concentration is uniform throughout the entire ion-conductivepolymer layer.

[0025] 1) The interfacial resistance between the cathode and theion-conductive layer is minimized and, therefore, the dischargecharacteristics at a high load is improved.

[0026] 2) The mobility of lithium ions is retarded in the sub-layer ofion-conductive polymer on the cathode to prevent the growth of lithiumdendrite from reaching the cathode.

[0027] 3) A concentration cell is formed within the battery and, thevoltage within the battery is elevated thereby. This enables a batteryhaving a high energy density to be provided.

[0028] In a preferred embodiment, the anodic electroactive substance isa graphite powder having amorphous carbon attached to the surfacesthereof. The use of this carbonaceous material in conjunction with thelaminate of ion-conductive polymer sub-layers prevents decrease in thedischarge capacity during repeated charge-discharge cycles. This may beattributed to retarded side reactions between the nonaqueous electrolytesolution and lithium formed upon charging.

[0029] The battery of the present invention may be manufactured byforming an ion-conductive polymer layer separately on a pre-fabricatedcathode and anode and joining the layers together although themanufacturing process is not limited thereto.

[0030] Examples of anodic electroactive substances include lithiummetal, a lithium-aluminum alloy, a lithium-lead alloy, a lithium-tinalloy, a lithium-aluminum-tin alloy, a lithium-gallium alloy, Wood'salloy and other alloys containing lithium but are not limited thereto.These anodic substances may be used alone or in combination.

[0031] It is also possible to use as the anodic electroactive substancea carbonaceous material capable of electrochemically inclusion andrelease of lithium such as graphite. More preferably, the carbonaceousmaterial is graphite particles having attached on the surfaces thereofamorphous carbon particles. These particles may be obtained by dippingthe graphite particles in a coal-based heavy oil such as coal tar orpitch or a petroleum-based heavy oil and heating recovered graphiteparticles to a temperature above the carbonizing temperature todecompose the heavy oil, if necessary, followed by milling. Suchtreatment significantly retards the decomposing reaction of thenonaqueous electrolyte solution and the lithium salt occurring at theanode during the charge cycle to enable the charge and discharge cyclelife to be improved and also the gas evolution due to the abovedecomposition reaction to be prevented.

[0032] Examples of the cathodic electroactive substances which areusable in the present invention include oxides of metals of group 4A and4B of the periodic chart such as TiS₂, SiO₂, or SnO; oxides of metals of5A and 5B of the periodic chart such as V₂O₅, V₆O₁₂, VOx, Nb₂O₅, Bi₂O₃or Sb₂O₃; oxides of metals of group 6A and 6B of the periodic chart suchas Cr O₃, Cr₂O₃, MoS₂, WO₃ or SeO₂; oxides of metals of group 7A such asMnO₂ or Mn₂O₃; oxides of metals of group 8 of the periodic chart such asFe₂O₃, FeO, Fe₃O₄, Ni₂O₃, NiO, CoS₂ or CoO; and a metal compound of thegeneral formula:

[0033] Li_(a)MX₂ or Li_(a)MNbX₂ wherein M and N are a metal of group 1to 8 of the periodic chart and X is a chacogen element such as oxygen orsulfur, such as lithium-cobalt composite oxide or lithium-manganesecomposite oxide, as well as a conductive polymer material such aspolypyrrole, polyaniline, polyparaphenylene, polyacetylene or polyaceneand a carbonaceous material of pseudographite structure but are notlimited thereto.

[0034] The cathodic electroactive substance to be used in the presentinvention in conjunction with the carbonaceous anodic active substanceis preferably selected from a composite oxide of laminar or spinelstructure represented by the formula:

Li_(a)(A)_(b)(B)_(c)O₂

[0035] wherein

[0036] A is a transition metal element;

[0037] B is an element selected from the group consisting of a non-metalor semi-metal element of group 3B, 4B and 5B of the periodic chart, analkaline earth metal, Zn, Cu and Ti;

[0038] a, b and c are numbers satisfying the following relationship:

0<a≦1.15

0.85≦b+c≦1.30, and

c>0

[0039] Typical examples of the composite oxides include LiCoO₂, LiNiO₂and LiCoxNi_(1-x)O₂ (0<x<1). Use of these compounds in conjunction witha carbonaceous material as a anodic electroactive substance isadvantageous in that the battery exhibits a practically acceptabledynamic voltage even when the voltage variation generated by chargingand discharging the carbonaceous material per se (about 1 volt vs.Li/Li⁺), and that lithium ions necessary for charging and dischargingthe battery are already contained in the form of, for example, LiCoO₂ orLiNiO₂ before assembling the battery.

[0040] The respective electroactive substances may be combined, wherenecessary, with a chemically stable conductor material such as graphite,carbon black, acetylene black, carbon fiber or conductive metal oxides.

[0041] The binder is selected among those thermoplastic resins which arechemically stable, soluble in a suitable solvent but hardly attackedwith the nonaqueous electrolyte solution. A variety of suchthermoplastic resins have been known. For example, polyvinylidenefluoride (PVDF) may preferably be used since this resin is selectivelysoluble in N-methyl-2-pyrrolidone.

[0042] The electrode may be produced by kneading the respectiveelectroactive substances and, where necessary, the conductor materialwith a solution of the binder resin to prepare a paste, applying thepaste on a metal foil using a suitable coater to form a film of uniformthickness, and compressing the film after drying. The proportion of thebinder resin in the electroactive substance layer should be minimum andgenerally lies from 1 to 15% by weight. The proportion of the conductormaterial usually lies, when used, from 2 to 15% by weight of theelectroactive substance layer.

[0043] The polymer electrolyte layer is formed on the respectiveelectroactive substance layers thus prepared integrally therewith. Thepolymer electrolyte layer is comprised of a matrix of an ion-conductivepolymer impregnated with or retaining a nonaqueous electrolyte solutioncontaining a lithium salt. The polymer electrolyte layer occursmacroscopically in a solid state but microscopically retains acontinuous phase of the lithium solution formed therein in situ. Thepolymer electrolyte layer of this type has an ion-conductivity higherthan that of the corresponding polymer electrolyte free from the lithiumsolution.

[0044] The polymer electrolyte layer may be formed by polymerizing (heatpolymerization, photopolymerization etc.,) a precursor monomer of theion-conductive polymer in the form of a mixture with the nonaqueouselectrolyte solution containing a lithium salt.

[0045] The monomer component which can be used for this purpose shouldinclude a polyether segment and also be polyfunctional in respect to thepolymerization site so that the resulting polymer forms a threedimensional crosslinked gel structure. Typically, such monomers may beprepared by esterifying the terminal hydroxyl groups with acrylic ormethacrylic acid (collestived called “(meth)acrylic acid”). As is wellknown in the art, polyether polyols are produced byaddition-polymerizing ethylene oxide (EO) alone or in combination withpropylene oxide (PO) using an initiator polyhydric alcohol such asethylene glycol, glycerine or trimethylolpropane. A monofunctionalpolyether polyol (meth)acrylate may be used in combination withpolyfunctional monomers.

[0046] The poly- and monofunctional monomers are typically representedby the following general formulas:

[0047] wherein

[0048] R₁ is hydrogen or methyl;

[0049] A₁, A₂ and A₃ are each a polyoxyalkylene chain containing atleast 3 ethylene oxide (EO) units and optionally some propylene oxide(PO) units such that PO/EO=0.25 and EO+PO≧35.

[0050] wherein

[0051] R₂ and R₃ are hydrogen or methyl;

[0052] A₄ is a polyoxyalkylene chain containing at least 3 EO units andoptionally some PO units such that PO/EO=0-5 and EO+PO≧=10.

[0053] wherein R₄ is a lower alkyl, R₅ is hydrogen or methyl, and A₅ isa polyoxyalkylene chain containing at least 3 EO units and optionallysome PO units such that PO/EO=0-5 and EO+PO≧3.

[0054] Non-limitative examples of the organic solvents include ethylenecarbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC),diethyl carbonate (DEC), γ-butyrolactone (GBL); and mixtures of thesesolvents. A mixed solvent of EC with another solvent is preferable. Itis also preferable to use different solvent mixtures between the cathodeside and the anode side, for example, a mixture of EC/GBL for the anodeside and a mixture of EC/EMC for the cathode side.

[0055] The nonaqueous electrolyte solution is prepared by dissolving alithium salt in the above solvent. Non-limitative examples of thelithium salt solutes include LiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiN(CF₃SO₂)₂and the like.

[0056] Since the ion-conductive polymer layer is of a laminate structureof sub-layers on the cathode side and the anode side, respectively, itis possible to use different lithium salts for both sub-layers, forexample, LiBF₄ for the anode side and LiPF₆ for the anode side.

[0057] According to the present invention, the concentration of lithiumsalt in the nonaqueous electrolyte solution contained in theion-conductive polymer is preferably at least 10% higher on the cathodeside than on the anode side. For example, the concentration may be from1.0 to 3.5 mol/L, preferably from 1.0 to 2.75 mol/L on the cathode sideand from 0.9 to 2.0 mol/L on the anode side. This improves variousperformance, particularly the discharge characteristics under heavyloads and the charge-discharge cycle characteristics of the resultingbattery.

[0058] Moreover, when the precursor monomer of ion-conductive polymerwas polymerized in the mixture with the nonaqueous electrolyte solutionhaving a lithium salt concentration in the above range, it was foundthat the amount of residual monomer on the cathode side was minimized.This is considered to be one of reasons why the interfacial resistancebetween the cathode and the ion-conductive polymer layer is decreased.

[0059] The proportion of the nonaqueous solution in the mixture with theprecursor monomer should be large enough to maintain the solution ascontinuous phase in the crosslinked polymer electrolyte layer but shouldnot be so excessive to undergo phase separation and bleeding of thesolution from the gel. This can be accomplished by the ratio of themonomer to the electrolyte solution generally within a range from 30/70to 2/98, preferably within a range from 20/80 to 2/98 by weight.

[0060] The monomer-electrolyte solution mixture may contain a suitablepolymerization initiator depending on the polymerization method, forexample, an initiator of peroxide or azo type in the heat polymerizationand a photoinitiator such as 2,2-dimethoxy-2-phenylacetophenone in thephotopolymerization. The polymer electrolyte layer may be formedintegrally with the respective electrodes by casting themonomer-nonaqueous electrolyte solution mixture on the electroactivesubstance layer of the respective electrodes as a film and polymerizingthe film by the heat polymerization or irradiating the film with UVradiation. When used, a separator is placed on either one of theelectrodes and the above procedure may be followed thereafter.Preferably, the separator is a porous membrane made of a polymer such aspolypropylene, polyethylene or polyester having an air-permeability from1 to 500 sec/cm³ although it is not limited thereto.

[0061] The battery of the present invention is assembled by joining thesub-layers of polymer electrolyte integrally formed with the respectiveelectrodes together.

EXAMPLE

[0062] The following Examples are for illustrative purpose only and notintended to limit the scope of the present invention thereto.

Example 1

[0063] 100 weight parts of graphite powder having amorphous carbonattached on the surfaces thereof was blended with 9 weight % ofpolyvinylidene fluoride (PVDF) as a binder. The blend was kneaded withan amount of N-methyl-2-pyrrolidone (NMP). The resulting paste wasapplied on a copper foil in a uniform thickness, dried and compressed toprepare an anode.

[0064] LiPF₆ was dissolved at a concentration of 1.0 mol/L in a mixedsolvent of ethylene carbonate (EC) and ethylmethyl carbonate at avolumetric ratio of 1:2 to prepare a nonaqueous electrolyte solution.

[0065] To the above nonaqueous electrolyte solution was mixed at aweight ratio of 90:10 a trifunctional polyether polyacrylate having amolecular weight of 7,500 to 9,000 of the formula:

[0066] wherein A₁, A₂ and A₃ are each polyoxyalkylene chain containingat least 3 EP units and at least one PO unit at PO/EO ratio of 0.25. Apolymerization solution was prepared by adding 1,000 ppm of2,2-dimethoxy-2-phenylacetophenone (DMPA).

[0067] This monomer/electrolyte solution mixture was cast on theelectroactive substance layer of the anode and then irradiated with UVradiation of 365 nm wavelength at an intensity of 30 mW/cm² for 3minutes' to polymerize the monomer in situ. An ion-conductive polymergel layer having a thickness of 20 μm was formed on the anode integrallytherewith.

[0068] 100 weight parts of LiCoO₂ powder, 5 weight % of PVDF binder and3 weight % of acetylene black conductor material were blended andkneaded with an amount of NMP. The resulting paste was applied on analuminum foil into a uniform thickness, dried and compressed to prepareda cathode.

[0069] Separately, LiPF₆ was dissolved at a concentration of 1.5 mol/Lin a mixed solvent of EC and γ-butyrolactone (GBL) at a volumetric ratioof 3:7 to prepare a nonaqueous electrolyte solution.

[0070] To the resulting electrolyte solution were added the abovetrifunctional polyether polyol polyacrylate at a weight ratio of97.5:2.5 and-500 ppm of DMPA to prepare a polymerization solution.

[0071] The resulting monomer/electrolyte solution mixture was cast onthe electroactive substance layer of the cathode and then irradiatedwith UV radiation of 365 nm wavelength at an intensity of 30 mW/cm² for3 minutes to polymerize the monomer in situ. An ion-conductive polymergel layer having a thickness of 20 μm was formed on the cathodeintegrally therewith.

[0072] Finally, the ion-conductive polymer layers integrally formed onthe respective electrodes were joined together to produce a battery.

Comparative Example 1

[0073] Example 1 was repeated except that the lithium salt concentraionof the electrolyte solution retained by the ion-conductive polymer gellayer on the cathode was changed to 1.0 mol/L.

[0074] The batteries of Example 1 and Comparative Example 1,respectively were charged at a constant current of 2.3 mA until thebattery voltage reached 4.1 V. After reaching this voltage level, chargewas continued at a constant voltage for 12 hours. Each battery wasdischarged at different current levels of 2.3 mA, 5 mA, 10 mA and 20 mAuntil the battery voltage decreased to 2.75 V. The results of thischarge-discharge test under the above conditions are shown in the graphof FIG. 1. The batteries of Example 1 and Comparative Example 1 weredischarged at a constant current of 10 mA. The discharge curves ofbatteries in this test are shown in the graph of FIG. 2.

[0075] As shown in FIG. 1, a remarkable difference was seen in thedischarge capacity between batteries when discharged at various currentlevels by setting the lithium salt concentration of the electrolytesolution retained by the ion-conductive polymer layer on the cathodehigher than the corresponding concentration of the electrolyte solutionretained by the ion-conductive polymer layer on the anode.

[0076] Also as shown in the discharge curves of FIG. 2, decease involtage immediately after discharge is smaller in the battery of Example1 than in the battery of Comparative Example 1 and the average dischargevoltage is higher in the battery of Example 1 than the battery ofComparative Example 1. These results suggest that the interfacialresistance between the cathode and the ion-conductive polymer layer islowered.

[0077] The effect of the difference in the lithium salt concentration ofthe electrolyte solution retained by the ion-conductive polymer layerformed on the cathode on the level of residual monomer in thepolymerized layer was studied. The monomer/electrolyte solution mixtureused in Example 1 (lithium salt=1.6 mol/L) and the mixture used inComparative Example 1 (lithium salt=1.0 mol/L), respectively cast on astainless steel foil and polymerized under the same conditions ofExample 1 and Comparative Example 1. Samples were taken from respectivepolymerized layers and assayed for the level of residual monomer by theGPC. The level of residual monomer was 4.2% for Example 1 and 7.1% forComparative Example 1. This suggests that the level of residual monomerrelates to the interfacial resistance between the cathode and theion-conductive layer formed thereon.

Example 2

[0078] A blend of 100 weight parts of natural graphite powder(Madagacar), 7 weight % of PVDF binder and 1 weight % of graphatizedcarbon black was kneaded with an amount of NMP. The resulting paste wascast on a copper foil uniformly, dried and compressed to prepare acathode.

[0079] LiN(CF₃SO₂)₂ was dissolved at a concentration of 1 mol/L in amixture of DC:DMC at a volumeric ratio of 1:2 to prepare an electrolytesolution. To this solution were mixed the same trifunctional polyetherpolyol polyacrylate as used in Example 1 at a weight-ratio of 90:10. Apolymerization solution was prepared by adding 1,000 ppm of DMPA to thismixture.

[0080] This monomer/electrolyte solution mixture was cast on theelectroactive substance layer of the anode and then irradiated with UVradiation of 365 nm wavelength at an intensity of 30 mW/cm² for 3minutes to polymerize the monomer in situ. An ion-conductive polymer gellayer having a thickness of 20 μm was formed on the anode integrallytherewith.

[0081] 100 weight parts of LiCo_(0.9)Ni_(0.1)O₂ powder, 5 weight % ofPVDF binder and 3 weight % of acetylene black conductor material wereblended and kneaded with an amount of NMP. The resulting paste wasapplied on an aluminum foil into a uniform thickness, dried andcompressed to prepared a cathode.

[0082] Separately, LiPF₆ was dissolved at a concentration of 1.5 mol/Lin a mixed solvent of EC and γ-butyrolactone (GBL) at a volumetric ratioof 3:7 to prepare a nonaqueous electrolyte solution.

[0083] To the resulting electrolyte solution were added the abovetrifunctional polyether polyol polyacrylate at a weight ratio of 95:5and 500 ppm of DMPA to prepare a polymerization solution.

[0084] The resulting monomer/electrolyte solution mixture was cast onthe electroactive substance layer of the cathode and then irradiatedwith UV radiation of 365 nm wavelength at an intensity of 30 mW/cm² for3 minutes to polymerize the monomer in situ. An ion-conductive polymergel layer having a thickness of 20 μm was formed on the cathodeintegrally therewith.

[0085] Finally, the ion-conductive polymer layers integrally formed onthe respective electrodes were joined together to produce a battery.

Example 3

[0086] Example 2 was repeated except that the graphite powder havingamorphous carbon attached on the surfaces thereof was used as an anodicelectroactive substance.

Comparative Exmaple 2

[0087] Example 2 was repeated except that the lithium salt concentrationof the electrolyte solution retained in the ion-conductive polymer gellayer was altered to 1 mol/L for the cathode and 2 mol/L for the anode,respectively.

[0088] The batteries of Examples 2, 3 and Comparative Example 2,respectively were charged at a constant current of 2.3 mA until thebattery voltage reached 4.1 V. After reaching this voltage, the chargewas continued at a constant voltage for 12 hours. Each battery wasdischarged at a constant current of 2.3 mA and the charge-dischargecycle under the above condition was repeated. The results are shown inthe graph of FIG. 3.

[0089] As shown in FIG. 3, the battery of Example 2 exhibited adischarge capacity higher than the battery of Comparative Example 2 whencompared at the same number of repeated charge-discharge cycles. Theprolonged charge-discharge cycle life is considered to be attributableto the elevated lithium salt concentration in the ion-conductive polymergel layer on the cathode compared to the corresponding lithium saltconcentration on the anode side.

[0090] Similarly, when comparison is made between the batteries ofExample 2 and Example 3, the battery of Example 3 retained a dischargecapacity higher than the battery of Example 2 at the same number ofrepeated charge-discharge cycles. The prolonged charge-discharge cyclelife is considered to be attributed the use of graphite carbonaceousmaterial having attached amorphous carbon instead of natural graphite assuch.

Example 4

[0091] In this Example, a battery was manufactured by the followingsteps of a)-e).

[0092] a) 100 weight parts of V₂O₅ as a cathodic electroactivesubstance, 180 weight parts of 3 wt. % solution of ethylene/1,3-cyclohexadiene copoymer in xylene as a binder solution, and 5 weightparts of acetylene black as a conductor material were kneaded. Theresulting paste was applied on a rolled aluminum foil, dried andcompressed to prepare a cathode.

[0093] b) LiBF₄ was dissolved in a mixture of EC:GBL:EMC at a volumetricratio of 35:35:30 at a concentration of 2.0 mol/L. To this solution wasadded the above functional polyether polyol polyacrylate at a weightratio of 95:5 followed by addition of 500 ppm of DMPA. The resultingmonomer/electrolyte solution was cast on the electroactive substancelayer of the cathode and irradiated with UV radiation of 365 nmwavelength at an intensity of 30 mW/cm² for 3 minutes to polymerize themonomer in situ. An ion-conductive polymer gel layer having a thicknessof 20 μm was formed on the cathode integrally therewith.

[0094] c) Lithium metal was used as an anodic electroactive substance byapplying lithium metal on a copper foil collector under pressure.

[0095] Then, a 1.0 mol/L solution of LiBF₄ in a mixed solvent ofEC:GBL:EMC at a volmetric ratio of 35:35:30 was prepared and the abovetrifunctional polyether polyol polyacrylate was added to the resultingelectrolyte solution at a weight ratio of 5:95. A polymerizationsolution was prepared by addition 500 ppm of DMPA to thismonomer/electrolyte solution mixture.

[0096] The resulting polymerization solution was cast on the lithiummetal layer of the anode and irradiated with UV radiation of 365 nmwavelength at an intensity of 30 mW/cm² for 3 minutes to polymerize themonomer in situ. An ion-conductuve polymer gel layer having a thicknessof 20 μm was formed on the anode integrally therewith.

[0097] d) The battery of Example 4 was produced by joining theion-conductive polymer layer/lithium/collector assembly prepared in stepc) and the collector/cathodic electroactive substancelayer/ion-conductive layer assembly prepared in step b) with the polymerlayers facing inwardly.

Comparative Example 3

[0098] Example 4 was repeated except that the lithium salt concentrationof the electrolyte solution retained in the ion-conductive polymer gellayer on the cathode was altered to 1.0 mol/L.

[0099] The batteries of Example 4 and Comparative Example 3,respectively were charged at a constant current of 2.3 mA until thebattery voltage reached 3.2 V and discharged at a constant current of2.3 mA until the battery voltage decreased to 2.0 V. The results of thischarge and discharge test are shown in FIG. 4.

[0100] As shown in FIG. 4 the battery of Example 4 is excellent in thecharge-discharge cycle characteristics compared to the battery ofComparative Example 3. This demonstrates that the charge-discharge cyclelife may be prolonged by elevating the lithium salt concentration in theion-conductive polymer gel layer to a higher level on the cathode sidethan on the anode side.

[0101] When comparison is made between FIG. 3 and FIG. 4, the use ofgraphite powder having amorphous carbon attached to the graphitesurfaces as a cathodic electroactive substance is advantageous overlithium metal in the charge-discharge cycle characteristics and,therefore, use of such a carbonaceous material is most preferable forthe battery of the present invention.

Example 5

[0102] Example 4 was repeated except that a polyester nonwoven fabrichaving a air-permeability of 150 sec/cm³ in step b) of Exmaple 4 and themonomer/electrolyte solution mixture was cast thereon. The totalthickness of the resulting ion-conductive polymer layer was 20 μm.

Comparative Example 4

[0103] The battery of Comparative Example 4 was manufactured by thefollowing steps a)-e).

[0104] a) Same as in Example 4 to prepare a cathode.

[0105] b) Example 4 was followed except that the lithium saltconcentration in the electrolyte solution was 1.0 mol/L. Anion-conductive polymer gel layer having a thickness of 1.0 μm was formedon the cathode integrally therewith.

[0106] c) Example 4 was followed except that the lithium saltconcentration in the electrolyte solution was 2.0 mol/L. Anion-conductive polymer gel layer of 10 μm thickness was formed on theanode integrally therewith.

[0107] d) The same polyester nonwoven fabric as used in Example 5 wasimpregnated with the monomer/electrolyte solution mixture prepared instep c) above. The impregnated fabric was sandwiched between a pair ofquartz plates and irradiated with UV radiation of 365 wavelength at anintensity of 30 mA/cm² for 3 minutes to form an ioin-conductive polymergel layer of 20 μm thickness integrally with the polyester nonwovenfabric separator.

[0108] e) The separator including the ion-conductive polymer prepared instep d) was sandwiched between the cathodecollector/anode/ion-conductive polymer layer assembly prepared in stepb) and the ion-conductive polymer layer/lithium/anode collector assemblyprepared in step c) and joined together with the ion-conductive polymerlayers facing the separator to prepare the battery of ComparativeExample 4.

[0109] The batteries of Example 5 and Comparative Example 4 were eachcharged at a constant current of 2.3 mA until the battery voltagereached 4.1 V. Thereafter, charge was continued at a constant voltagefor 12 hours. Then each battery was discharged at different constantcurrent levels of 2.3 mA, 5 mA, 10 mA and 20 mA until the batteryvoltage decreased to 2.75 V. The results of the charge-discharge testunder the above conditions are shown in FIG. 5.

[0110] As shown in FIG. 5, a remarkable difference in the dischargecapacity at different current levels is seen between the battery havingtwo ion-conductive polymer layers (Example 5) and the battery havingthree ion-conductive polymer layers (Comparative Example 4). It is alsoobserved that the battery capacity upon discharge at a hight currentlevel may not be improved unless the litnium salt concentration in theion-conductive polymer layer is higher on the cathode side than on theanode side regardless of the lithium salt concentration in theion-conductive polymer retained in the separator.

1. A lithium secondary battery comprising an anode for lithiumbatteries, a cathode containing lithium or capable of inclusion andrelease of lithium, and ion-conductive polymer layer sandwiched betweenthe cathode and the anode, wherein said ion-conductive layer is alaminate of two sub-layers each comprising a matrix of saidion-conductive polymer retaining a nonaqueous electrolyte solutiontherein, and wherein said sub-layers are integral with the respectiveelectrodes and the concentration of a lithium is higher in saidnonaqueous solution retained by the sub-layer on the cathode side thanin said nonaqueous electrolyate solution retained in the sub-layer onthe anode side.
 2. The lithium secondary battery according to claim 1wherein the electroactive substance of said anode is a carbonaceousmaterial capable of electrochemically inclusion and release of lithium,and the electroactive substance of said cathode is a chalcogenidecompound containing lithium.
 3. The lithium secondary battery accordingto claim 1 wherein said lithium salt concentration in said nonaqueouselectrolyte solution is at least 10% higher on the cathode side than onthe anode side.
 4. The lithium secondary battery according to claim 1wherein said lithium salt concentration in the nonaqueous electrolytesolution retained in the ion-conductive poymer sub-layer is 1.0 to 3.5mol/L on the cathode side and 0.8 to 2.2 mol/L on the anode side.
 5. Thelithium secondary battery according to claim 1 wherein said matrix ofion-conductive polymer is a homo- or copolymer of polyether polyolpoly(meth)acrylate including an ethylene oxide unit and optionally apropylene oxide unit in the polyether chain.
 6. The lithium secondarybattery according to claim 2 wherein said carbonaceous material isgraphite powder having amorphous carbon attached to the surfacesthereof.
 7. The lithium secondary battery according to claim 1 whereinthe organic solvent of said nonaqueous electrolyte solution is selectedfrom the group consisting of ethylene carbonate, propylene carbonate,ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate,γ-butyrolactone and a mixture thereof.
 8. The lithium secondary batteryaccording to claim 1 wherein the organic solvent and/or the lithium saltin said nonaqueous electrolyte solution between the cathode side and theanode side in kind and/or composion.
 9. The lithium secondary batteryaccording to claim 1 wherein the ion-conductive polymer sub-layer eitheron the anode side or the cathode side includes a separator and acrosslinked ion-conductive polymer in the separator which are integralwith said sub-layer.